docs: def get_model(self, sm_id):
          """
          Extract a CompositeSourceModel instance containing the single
          model of index `sm_id`.
          """
          sm = self.source_models[sm_id]
          if self.source_model_lt.num_samples:
              self.source_model_lt.num_samples = sm.samples
          new = self.__class__(self.gsim_lt, self.source_model_lt, [sm],
                               self.optimize_same_id)
          new.sm_id = sm_id
          return new